Audio stream output by earphone can be viewed as the superposition of many sine waves of different frequencies; low-frequency enhancement is to improve the sound pressure level of low-frequency components in the audio stream by filtering and other methods, so that the voice sounds more vigorous.
The low-frequency enhancement in existing technologies mainly adopts filter technologies, and combines different filters and other components to meet different requirements. However, adopting pure filter technologies to perform low-frequency enhancement has certain limitation: it is cumbersome to adjust voice volume (actually to amplify/reduce the amplitude of the waveform of an audio signal so as to change the sound pressure) in the normal use of earphone, it can be known from the description of an equal-loudness contour that pure tones of different frequencies have different loudness at different sound pressure levels; therefore, in actual application, when low-frequency enhancement is performed on the audio stream output by an earphone, different gains need to be added to signals of different frequencies at different sound pressure levels, so that gains of signals of different frequencies in the output audio signal all meet the tendency of the equal-loudness contour when voice volume is adjusted and an optimal low-frequency enhancement effect is achieved; however, this requirement cannot be met in static filter combination in existing technologies.